1. Field of the Invention
The present invention relates to an optical detection element having radiation tolerance and a solid-state image pickup device having the radiation tolerance, in the solid-state image pickup device a number of pixels are arranged on a semiconductor chip using the optical detection element as the pixel.
2. Description of the Related Art
In pixels of a three-transistor CMOS solid-state image pickup device (hereinafter abbreviated as “3T type”), a photodiode is implemented by a p-n junction including a p-type semiconductor substrate and an n-type region disposed on the p-type semiconductor substrate. The n-type region sometimes has a double-layer structure including a buried n-type layer (n-well) having a low impurity concentration and an n-type layer having a high impurity concentration provided on the buried n-type layer. Since a pixel isolation insulating film is disposed in a peripheral portion of the photodiode and the pixel isolation insulating film is covered by a p-well, the p-well is inserted between the photodiode and the pixel isolation insulating film. Furthermore, a thick interlayer insulating film is provided on an upper surface of the photodiode and a wiring metal is disposed with the interlayer insulating film interposed between the photodiode and the wiring metal.
When gamma rays are irradiated to the 3T pixel, since the interface is activated and becomes the source of a large dark current, the dark current increases. The reason of the increment in the dark current is speculated as follows: when hydrogens are ionized by irradiation of gamma rays in a thick oxide film implementing the interlayer insulating film and the pixel isolation insulating film, because the ionized hydrogens diffuse through the oxide film to reach a semiconductor interface so that hydrogen gas is emitted, the hydrogen atoms suppressing the dark current on the interface are deprived.
Thus, a guard ring scheme, by which the periphery of a photodiode is surrounded by a MOS-type gate electrode and the outer side of the gate electrode is surrounded by a p-type guard ring region, is known as disclosed by U.S. Pat. No. 6,690,074. By applying a voltage, at a level by which the semiconductor surface will not be depleted, to the MOS-type gate electrode, the generation of a dark current in the peripheral portion of the photodiode can be suppressed. However, even by the invention disclosed in U.S. Pat. No. 6,690,074, since an issue of dark current, which will be generated in the interlayer insulating film disposed on the upper surface of the photodiode, still remains, the dark current increases with the amount of irradiation of gamma rays as illustrated in FIG. 13.
In pixels of a four-transistor CMOS solid-state image pickup device (hereinafter abbreviated as “4T type”) in which a transistor having a transfer-gate electrode is added to the 3T structure, a structure in which a p-type pinning layer having a high impurity concentration is formed on a buried n-type layer is often employed for eliminating the influence of surface states. However, even when the p-type pinning layer is formed, the dark current increases abruptly with irradiation of gamma rays of 1 kGy or higher as illustrated in FIG. 13. It is predicted that, when gamma rays are irradiated, since an electron-hole pair is generated in a thick oxide film, slow holes remain, and excessive positive charges remain in the oxide film, the shielding effect of the p-type pinning layer disappears.
In any of the earlier 3T and 4T CMOS solid-state image pickup devices, the capacitance of the photodiode is large and the charge-voltage conversion gain is low. Therefore, there is a problem that the earlier CMOS solid-state image pickup device has very low voltage-sensitivity.